1. Field of the Invention
The present invention relates to a semiconductor device and a manufacturing method thereof.
Note that in this specification, a semiconductor device refers to any device that can function by utilizing semiconductor characteristics, and an electro-optical device, a semiconductor circuit, an electronic device, and the like are all semiconductor devices.
2. Description of the Related Art
A technique for forming a transistor by using a semiconductor film formed over a substrate having an insulating surface has attracted attention. The transistor is applied to a wide range of semiconductor devices such as an integrated circuit (IC) and an image display device (display device). A silicon-based semiconductor film is widely known as a semiconductor film applicable to the transistor. In recent years, an oxide-based semiconductor film has attracted attention.
For example, a transistor is disclosed in which an amorphous oxide film containing indium, gallium, and zinc and having an electron carrier concentration of lower than 1018/cm3 is used (see Patent Document 1).
A transistor including an oxide semiconductor film has high electron mobility in the oxide semiconductor film as compared to a transistor including an amorphous silicon film; thus, operation speed thereof can be drastically increased. In addition, there is an advantage that capital investment can be reduced because part of production equipment for a transistor including an amorphous silicon film can be retrofitted and utilized.